The search for kinase inhibitors has proven to be a fruitful area for the development of useful pharmaceutically active substances. Kinases, which are alternatively known as phosphotransferases, are enzymes that transfer phosphate groups from high energy donor molecules (for example ATP) to specific target molecules (typically called substrates) in a process termed phosphorylation. One of the largest groups of kinases are the protein kinases which act on and modify the activity of specific proteins.
As a result of this activity these kinases are involved in a number of cellular processes such as in signalling and to prime the cell for biochemical reactions in metabolism. Certain cellular signalling processes have been implicated as important in a number of medical conditions and the effective inhibition of certain cell signalling processes therefore provides the potential to stop these conditions developing. Accordingly, kinases represent an attractive target for medicinal chemists as the provision of kinase inhibitors potentially allows for certain signalling processes to be controlled leading to the control of certain medical conditions.
One family of kinases associated with undesirable medical conditions in the body are the phosphoinositide 3-kinase (PI3) family of kinases which are involved in a wide range of cellular events such as cell migration, cell proliferation, oncogenic transformation, cell survival, signal transduction and intracellular trafficking of proteins. This family of kinases has recently been the focus of much research aimed at developing therapies for a range of indications such as proliferative diseases, for example cancer, immune and inflammatory diseases, diseases supported by excessive neovascularization and transplant rejection.
The phosphoinositide 3-kinase (PI3K) family is a group of enzymes that generate phosphatidylinositol ‘second messengers’. These lipids are subsequently involved in a wide range of physiological processes. In mammalian cells, the large PI3K family has been categorized into three classes, referred to as I, II, and III, each of which has its own characteristics in terms of molecular structure and substrate specificity. Class I PI3K preferred in vivo substrate is phosphatidylinositol-4,5 bisphosphate, which is phosphorylated to yield phosphatidylinositol-3,4,5 trisphosphate. These are further subdivided into Class IA and IB PI3Ks. Class IA enzymes consist of any one of the ‘catalytic’ subunits (p110α, p110β, or p110δ) complexed with any one of the ‘regulatory’ subunits (p85α, p85β or p55γ). Only one Class IB PI3K enzyme exists, and is made up of the p110γ catalytic and the p101 regulatory subunit. There are also three Class II PI3Ks (CIIα, CIIβ, and CIIδ) and one Class III PI3K (Vps34).
The class I PI3Ks are the best understood members of this family and are key players of multiple intracellular signalling networks that integrate a variety of signals initiated by many growth factors. The Class IA enzymes are activated by tyrosine kinases (e.g. growth factor receptors), antigen receptors, and cytokine receptors, whilst the Class IB enzyme is activated by ‘G Protein Coupled Receptors’ (GPCRs). In response to activation, the PI3Ks generate lipid second messengers, which bind to, and activate, specific proteins in distinct signal transduction pathways. The signal transduction pathways remain active until phosphatase enzymes, in particular the oncogene PTEN, dephosphorylate the PI3K lipid second messengers.
The PI3K signalling pathway is crucial to many aspects of cell growth and survival via its regulation of widely divergent physiological processes that include cell cycle progression, differentiation, transcription, translation and apoptosis. Constitutive activation of the PI3K pathway has been implicated in both the pathogenesis and progression of a large variety of cancers and there is now a rapidly accumulating body of evidence that demonstrates conclusively that PI3K signalling is frequently deregulated in cancer. The deregulation of PI3K signalling is thought to occur in two different ways. The first is an increase in PI3K signalling resulting from activating gene mutations, amplification and over expression of PI3Ks or upstream receptors that activate PI3Ks. For example, the PI3Kα catalytic subunit is amplified and over expressed in ovarian and cervical cancers. Similarly, upstream receptor tyrosine kinases that activate PI3K are commonly mutated, amplified and over expressed, e.g., EGFR in breast, ovarian and lung cancer.
In addition, activation of the effectors downstream of PI3K can also contribute to deregulation of the PI3K pathway, e.g., Akt/PKB (Protein Kinase B) is over expressed and activated in breast, pancreatic and ovarian cancers among others. Also, the Ras family members, which are involved in PI3K activation, are frequently mutated, e.g. in colorectal and pancreatic cancer. The second mechanism of PI3K deregulation involves loss of the tumor suppressor phosphatase PTEN, which occurs in many aggressive brain tumors, endometrial and breast cancers, and melanomas.
One specific cell signalling pathway mediated by the PI3 family of kinases is the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. This pathway is critically involved in the mediation of cell survival and is a major signalling component downstream of growth factor receptor tyrosine kinases (RTKs). Growth factor RTKs engage the class-IA PI3K, which is a heterodimer comprised of the p85 regulatory and p110 catalytic subunits. The small GTPase Ras can also recruit and activate PI3K through direct binding to p110. At the cell membrane, PI3K catalyzes the production of the lipid second messenger phosphatidylinositol-3,4,5-triphosphate (PIP3). Subsequently, PIP3 recruits other downstream molecules—particularly the serine-threonine kinases Akt and PDK1—via binding to their pleckstrin-homology (PH) domains. At the membrane, Akt is partially activated through phosphorylation at threonine 308 in its activation loop by PDK1. Additional phosphorylation at serine 473 in the C terminus of Akt results in its full activation. Akt in turn regulates a wide range of target proteins, one of which is the mammalian target of Rapamycin (commonly known as mTOR). The levels of PIP3 in the cell are strictly regulated and several lipid phosphatases act to rapidly remove it. Of particular interest is the phosphatase PTEN, which converts PIP3 back to PIP2 and thus shuts off PI3K signalling. The PI3K-Akt signalling pathway regulates many normal cellular processes including cell proliferation, survival, growth, and motility—processes that are critical for tumorigenesis.
The role of the PI3K/Akt pathway in oncogenesis has also been extensively investigated and mutations or altered expression of most of the pathway's components have been widely implicated in many cancers. Gene amplification of p110 occurs in some cases of human ovarian cancer, and amplification of Akt is found in ovarian, breast, and colon cancer. In addition, activating mutations in p85 have been identified in ovarian and colon cancer. Most importantly PTEN has been identified as a major tumor suppressor in humans and loss-of-function mutations in the PTEN gene are extremely common among sporadic glioblastomas, melanomas, prostate cancers, and endometrial carcinomas, and a significant percentage of breast tumors, lung cancers, and lymphomas also bear PTEN mutations. Thus, through a variety of mechanisms, a high percentage of human cancers possess activated PI3K signalling. Significantly, it has been shown that mTOR is important for the oncogenic transformation induced by PI3K and Akt.
In addition to the compelling correlative data presented above, direct proof of the involvement of deregulated PI3K signalling in cancer comes from mouse genetic models. For example, mice with a constitutively activated p85 regulatory subunit of PI3K progress to malignant lymphoma when crossed with p53-knockout mice. Further, retroviral introduction of Akt and Ras caused glioblastomas in mice. Taken together, all these data provide strong validation for the development of novel anticancer strategies targeted at PI3Ks. Indeed recent interest in PI3K inhibitors has been intense with a number of compounds now in development having demonstrated anti-tumor activity in animal models. The most advanced compounds are now undergoing evaluation in phase I clinical trials. Accordingly compounds that are PI3K inhibitors would be expected to show interesting biological activity as PI3K inhibitors have the potential to block the PI3K/Akt signalling pathway and thereby form the basis of therapy in disease involving deregulation of this pathway.
In addition, PI 3-kinase isoforms p110δ and p110γ regulate different aspects of immune and inflammatory responses. Hence there is great interest in the role of PI3-kinase signaling in a range of immune and inflammatory diseases as well as in transplant rejection.
Another area that has received attention has been the serine/threonine kinases. One serine/threonine kinase that has attracted significant interest is mTOR.
mTOR is a serine/threonine kinase of 289 kDa and is a PI3K-like kinase that links mitogenic stimuli and nutrient status to cell growth and division. mTOR was discovered during studies conducted to understand the mechanism of action of rapamycin. Upon entering cells, rapamycin binds to its intracellular target FKBP12 and the complex then binds to and specifically inhibits mTOR. mTOR was, therefore, also named FKBP-RAP associated protein (FRAP), RAP FKBP12 target (RAFT1) and RAP target (RAPT1). Cells responsible for organ rejection stop growing due to rapamycin's ability to inhibit the anabolic signals coordinated by mTOR. Since inhibition of cell growth represents a valid target for treating cancer, designing new drugs that inhibit mTOR will potentially have therapeutic value.
In humans, mTOR mediates anabolic signals from 2 sources namely nutrients that pass into the cell and activated growth factor receptors. It exists in at least two distinct complexes: a rapamycin-sensitive complex, referred to as mTOR complex 1 (mTORC1), defined by its interaction with the accessory protein raptor (regulatory-associated protein of mTOR). The normal activation of mTOR results in an increase in protein translation because mTORC1 phosphorylates and activates the translation regulators eukaryotic initiation factor 4E-binding protein 1 and ribosomal p70 S6 kinase. Therefore, by inhibiting mTOR, rapamycin causes a decrease in phosphorylation of these effectors, and a decrease in protein synthesis, effectively blocking the pro-growth actions of mTOR.
The second complex, mTOR complex 2 (mTORC2), is rapamycin-insensitive and is defined by its interaction with rictor (rapamycin-insensitive companion of mTOR). mTORC2 is involved in the regulation of the pro-survival kinase Akt/PKB by phosphorylating it on S473. Together with the phosphorylation of T308 by PDK1, S473 phosphorylation is necessary for full Akt activation. Recent reports indicate that prolonged treatment with rapamycin in some cells also suppresses the assembly and function of TORC2 to inhibit Akt and that this property of rapamycin contributes to the anti-apoptotic effects of the drug. mTOR is also one of the main downstream effectors in the phosphatidylinositol 3-kinase (PI3K)/Akt pathway and therefore inhibition of mTOR provides a further opportunity to inhibit, at least in part, the PI3K/Akt pathway.
An additional pathway influenced by mTOR that appears to be particularly important in renal cell carcinoma involves the hypoxia-inducible factor (HIF). With loss of Von Hippel-Lindau (VHL) gene function commonly seen in clear cell renal cell cancer, there is accumulation of the oxygen-sensitive transcription factors HIF-1 and HIF-2. An accumulation of these factors yields increased stimulation of vascular endothelial growth factor (VEGF), platelet-derived growth factor, and transforming growth factor. This effect is augmented by the activation of mTOR, which stimulates both a protein stabilization function and a protein translational function and, thus, increases HIF-1 activity.
It has also been determined that tuberous sclerosis complex gene products, TSC1 and TSC2, function together to inhibit mTOR-mediated downstream signalling. Mutations of these genes occur in tuberous sclerosis and their loss of function yields yet another pathway, which leads to increased activity of mTOR and induces VEGF production. TSC2 also regulates HIF. Thus, studies evaluating the impact of TSC1 and TSC2 mutations demonstrate the connection of increased VEGF and activated mTOR pathways to angiogenesis.
So far, four mTOR inhibitors have been tested in clinical trials: the prototype rapamycin and three rapamycin derivatives, CCI-779 (temsirolimus), RAD001 (everolimus) and AP23573. Rapamycin, also named sirolimus, is a natural antibiotic produced by Streptomyces hygroscopicus. It was developed initially as an anti-fungal drug directed against Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. Later, rapamycin was developed as an immunosuppressive agent and those studies helped in understanding the mechanism of action of this agent. As an anti-cancer agent, rapamycin was shown to inhibit the growth of several murine and human cancer cell lines in a concentration-dependent manner, both in tissue culture and xenograft models. In the sixty tumor cell lines screened at the National Cancer Institute in the USA, general sensitivity to the drug was seen at doses under 2000 ng/ml, more evident in leukemia, ovarian, breast, central nervous system and small cell lung cancer cell lines. In addition, rapamycin inhibits the oncogenic transformation of human cells induced by either PI3K or Akt and has shown metastatic tumor growth inhibition and anti-angiogenic effects in in vivo mouse models.
Based on these pre-clinical results, clinical trials with rapamycin as an anticancer drug were carried out and rapamycin analogues with more favourable pharmaceutical properties were developed. CCI-779, a more water-soluble ester derivative of rapamycin was identified by investigators at Wyeth Ayerst as a non-cytotoxic agent that delayed tumor cell proliferation. At several non-toxic doses, CCI-779 demonstrated anti-tumor activity alone or in combination with cytotoxic agents in a variety of human cancer models such as gliomas, rhabdomyosarcoma, primitive neuroectodermal tumor such as medulloblastoma, head and neck, prostate, pancreatic and breast cancer cells. Treatment of mice with CCI-779 inhibits p70S6K activity and reduces neoplastic proliferation. As with rapamycin, PTEN-deficient human tumors are more sensitive to CCI-779-mediated growth inhibition than PTEN expressing cells. Specifically, studies in vitro in a panel of eight human breast cancer cell lines showed that six of eight cancer lines studied were inhibited by CCI-779 with IC50 in the low nanomolar range. Two lines, however, were found to be resistant with IC50>1 μM. The sensitive cell lines were estrogen receptor positive or over-expressed HER-2/Neu, or had lost the tumor suppressor gene product PTEN. The main toxicities of CCI-779 included dermatological toxicities and mild myelosuppression (mainly thrombocytemia).
RAD001, 40-O-(2-hydroxyethyl)-rapamycin, is another analogue of rapamycin that can be administrated orally. Its anti-neoplastic activity has been evaluated in different human cancer cell lines in vitro and in xenograft models in vivo with IC50 ranging from 5 to 1800 nM. p70S6K inhibition and anti-neoplastic effects have been shown in these models, with an optimal effect being achieved with 2.5 mg/kg/day in melanoma, lung, pancreas and colon carcinoma. Similarly, RAD001 demonstrated a concentration-dependent anti-tumor activity in a syngenic rat pancreas carcinoma model with an intermittent dosing schedule. RAD001 has also shown anti-angiogenic activity and inhibits human vascular endothelial cell (HUVEC) proliferation. The toxicity reported for RAD001 includes hypercholesterolemia, hypertriglyceridemia, mild leukocytopenia and thrombocytopenia. In a phase I trial performed in patients with advanced cancer, RAD001 displayed a good safety profile with mild to moderate skin and mucous toxicity up to 30 mg weekly. Preliminary efficacy results showed an objective response in a patient with non-small cell lung carcinoma.
AP23573 is the latest rapamycin analog to be reported in clinical development. It is a phosphorus-containing compound synthesized with the aid of computational modelling studies. AP23573 was found to be stable in organic solvents, aqueous solutions at a variety of pHs and in plasma and whole blood, both in vitro and in vivo and has shown potent inhibition of diverse human tumor cell lines in vitro and as xenografts implanted into nude mice, alone or in combination with cytotoxic or targeted agents. In phase I trials, AP23573 was administered intravenously daily for 5 days every 2 weeks. Dose-limiting toxicity is severe grade 3 oral mucositis occurring during the first cycle. Other side effects seem to be moderate, including minor to moderate episodes of mucositis, fatigue, nausea, rash, anaemia, neutropenia, diarrhoea, hyperlipidemias and thrombocytopenia. Preliminary anti-tumor activity is observed at all dose levels.
There is thus a plethora of studies that demonstrate that mTOR inhibitors can improve cancer patient survival. However, rapamycin and its analogues have not shown universal anti-tumor activity in early clinical trials. Response rates vary among cancer types from a low of less than 10% in patients with glioblastomas and advanced renal-cell cancer to a high of around 40% in patients with mantle-cell lymphoma. Knowledge of the status of PTEN and PI3K/Akt/mTOR-linked pathways might help in the selection of tumor types that will respond to mTOR inhibitors. Furthermore, because many tumor types still do not respond to single agent therapy with rapamycin derivatives, it is important to continue the search for factors predictive of resistance or sensitivity to mTOR inhibitors. Of particular interest will be molecules that directly inhibit mTOR kinase activity, the assumption being that such molecules will inhibit both mTORC1 and mTORC2. Such an inhibitor might be beneficial for treating tumors with elevated Akt phosphorylation and might down-regulate the growth, proliferation and survival effects that are associated with Akt activation. If mTOR-rictor is a crucial activator of Akt-dependent survival processes, such a drug might promote apoptosis in tumor cells that have adapted to Akt-dependent regulatory mechanisms.
In addition mTOR inhibitors have been shown to be very effective in preventing organ rejection after transplantation through an effect on immune responses, demonstrating a potential for treatment of autoimmune and inflammatory diseases as well as cancer.
Through the role of PI3 K isoforms as key components of the down stream signalling pathways of angiogenic growth factors such as VEGF, FGF and PDGF as well angiogenic cytokines and because of the role of mTOR in the regulation of vascular endothelial growth factor (VEGF), PI3 K and mTOR inhibitors also have potential to treat diseases supported by pathological neovascularization. This occurs during tumorigenesis, inflammatory conditions such as rheumatoid arthritis and ocular neovascular diseases e.g., age-related macular degeneration (AMD), retinal vascular diseases (vein occlusion and diabetic retinopathy) and other possible proliferative vascular disorders.
mTOR and PI3 have been identified as protein kinases that are involved in a number of disorders, and compounds that target one or more of these kinases should display useful biological activity. Accordingly, compounds that are mTOR and/or PI3K inhibitors have the potential to provide further biologically active compounds that would be expected to have useful, improved pharmaceutical properties in the treatment of proliferative disorders such as cancer, immune and inflammatory diseases, diseases supported by excessive neovascularisation and organ transplant rejection.
Compounds that inhibit both mTOR and PI3K simultaneously may be expected to provide powerful anti-proliferative, anti-angiogenic and antitumor activity since these compounds act at multiple points in the PI3K/Akt/mTOR pathway. A number of inhibitors of this type are now being investigated in a clinical setting for the first time (e.g. BEZ235, XL765, GDC0941, PX866, SF1126).